This invention relates to a helical antenna typically mounted on a mobile terminal equipment for mobile communication and, in particular, to a two-resonance helical antenna.
A two-resonance helical antenna comprises a conductive holder having a threaded portion serving as a feeding portion, a pair of helical coils made of a conductive material and different in bore size or inner diameter from each other, and a pair of nonconductive guides made of a dielectric material and different in inner diameter from each other. The helical coils are smaller and greater in inner diameter and may be called a smaller helical coil and a greater helical coil, respectively. Likewise, the nonconductive guides are smaller and greater in inner diameter and may be called a smaller guide and a greater guide, respectively. The helical coils are connected to the conductive holder through the nonconductive guides, respectively, and arranged in a coaxial fashion. The nonconductive guides serve to prevent the deformation and the unstableness of the helical coils. Finally, a combination of the helical coils and the nonconductive guides is covered with a nonconductive cover.
In the two-resonance helical antenna thus assembled, the greater helical coil is fitted onto an outer peripheral surface of the greater guide of a cylindrical shape. Inside an inner peripheral surface of the greater guide, the smaller guide of a rod-like shape is arranged with the smaller helical coil fitted on its outer peripheral surface. The two helical coils are different in electrical length. The greater helical coil as an outer helical coil carries a lower resonance frequency as a first resonance frequency while the smaller helical coil as an inner helical coil carries a higher resonance frequency as a second resonance frequency.
The two-resonance helical antenna of the above-mentioned structure has several limitations imposed upon its design.
At first, in order to utilize the characteristic of the two helical coils lower in height than a linear conductor, the inner helical coil is required to have a relatively large inner diameter. Therefore, the outer helical coil is inevitably increased in inner diameter.
Second, the two helical coils are connected in parallel and arranged in a coaxial fashion. This is a bar to reduction in size of the antenna as a whole because the sizes of the helical coils (particularly, the size of the inner helical coil) are limited due to the above-mentioned arrangement.
Third, since the two helical coils overlap each other, the helical coils interfere with each other in their electric characteristics. Therefore, a resulting electric characteristic is different from that obtained by either one of the helical coils. If a parameter of one of the helical coils is changed, both of the first and the second resonance frequencies will be changed. Accordingly, in order to tune these frequencies with a desired frequency band, it is required to simultaneously adjust parameters of the two helical coils. This means that the variation in shape of the two helical coils gives a double influence upon the electric characteristic. Therefore, such variation in shape must be suppressed as much as possible.
However, the two-resonance helical antenna in the previous technique has a basic structure that the helical coils are arranged in a coaxial fashion to overlap each other. Therefore, the sizes of the helical coils are restricted and only a small degree of freedom is allowed. In addition, the reduction in size of the antenna as a whole is limited. Furthermore, the helical coils interfere with each other so that the variation in their shapes results in wide fluctuation in electric characteristic. Thus, the two-resonance helical antenna has various disadvantages in its structure.